1. Field of the Invention
The present invention relates to a handheld type four-cycle engine which is mainly used as a power source for portable working apparatus such as a trimmer. More particularly, it relates to improvement of the so-called OHV engine that includes an engine main body, the engine main body including a crankcase having a crank chamber, a cylinder block having a cylinder bore, and a cylinder head having an intake port and an exhaust port; a crankshaft supported in the crankcase and housed inside the crank chamber; a piston fitted in the cylinder bore and connected to the crankshaft; an intake valve and an exhaust valve for opening and closing the intake port and exhaust port, the intake valve and exhaust valve being mounted in the cylinder head; and a valve operation mechanism operable in association with the rotation of the crankshaft so as to open and close the intake valve and exhaust valve.
2. Description of the Related Art
As such an OHV engine which is already known, for example, Japanese Patent Application Laid-open No. 10-288019 discloses one in which an oil reservoir is provided in the lower part of a crankcase, an oil mist is generated by scattering the oil stored in the oil reservoir by the rotation of a crankshaft, and the inside of the engine is lubricated with the oil mist.
Generally, OHV engines having intake and exhaust valves in their cylinder head tend to be large in overall height due to the presence of the intake and exhaust valves and a valve operation mechanism for opening and closing them. However, an oil reservoir is formed in the lower part of the crankcase as in the above-mentioned conventional engine, the overall height is further increased and it becomes difficult to make the engine more compact.
The present invention has been carried out in view of the above-mentioned circumstances. It is an object of the present invention to lubricate the inside of the crank chamber and the valve operation mechanism reliably regardless of the operational position of the engine, while reducing the overall height of the engine so making it more compact.
In accordance with a first aspect of the present invention in order to achieve the above-mentioned objective, there is proposed a handheld type four-cycle engine including an engine main body, the engine main body including a crankcase having a crank chamber, a cylinder block having a cylinder bore, and a cylinder head having an intake port and an exhaust port; a crankshaft supported in the crankcase and housed inside the crank chamber; a piston fitted inside the cylinder bore and connected to the crankshaft; an intake valve and an exhaust valve for opening and closing the intake port and exhaust port, the intake valve and the exhaust valve being mounted in the cylinder head; and a valve operation mechanism operable in association with the rotation of the crankshaft so as to open and close the intake valve and the exhaust valve, wherein an oil tank for storing oil is provided so as to be connected to one side wall running the length of the crankcase and the cylinder block; the oil tank houses oil mist generation means for generating an oil mist from the stored oil, and a rotational movement section of the valve operation mechanism; the oil tank and the crank chamber are communicated with each other above the stored oil in the oil tank; the crank chamber and a valve operation chamber formed in the cylinder head so as to house a reciprocating movement section of the valve operation mechanism are communicated with each other via an oil feed passage; the valve operation chamber and the oil tank are communicated with each other above the stored oil in the oil tank via an oil return passage; and transfer means for sending only the positive pressure component of pressure pulsations generated in the crank chamber towards the valve operation chamber is provided in the oil feed passage. The above-mentioned transfer means corresponds to the one-way valve 51 in the embodiments of the present invention below.
In accordance with the above-mentioned first characteristic, since the oil tank is provided so as to be connected to one side wall running the length of the crankcase and the cylinder block, it is unnecessary to provide an oil reservoir in the lower part of the crankcase, and the overall height of the engine can thus be reduced and the engine can be made more compact.
Furthermore, since the oil tank is filled with the oil mist generated by the oil mist generation means, the rotational section of the valve operation mechanism provided inside the oil tank can be lubricated with the oil mist particularly well.
Moreover, since the oil mist inside the oil tank is supplied to the crank chamber and the valve operation chamber, and returned to the oil tank by utilising the pressure pulsations of the crank chamber, the inside of the crank chamber and the reciprocating movement section of the valve operation mechanism can be lubricated regardless of the operational position of the engine, and it is unnecessary to employ a special oil pump for circulating the oil mist so simplifying the structure.
In accordance with a second aspect of the present invention, in addition to the above-mentioned characteristic, there is proposed a handheld type four-cycle engine wherein an oil mist is generated by the motion of the rotational movement section of the valve operation mechanism scattering the stored oil inside the oil tank.
In accordance with the above-mentioned second characteristic, since the rotational section of the valve operation chamber functions as part of the oil mist generation means, the oil mist generation means can be obtained in a simple manner.
The above-mentioned object, other objects, characteristics and advantages of the present invention will become apparent from an explanation of preferable embodiments which will be described in detail below by reference to the attached drawings.